Heretofore, various line printers of the type described above have been developed. However, the prior art line printers in general utilize a plurality of printing rings coaxially arranged on a driving shaft adjacent to each other with a spring interposed between the respective printing ring and the driving shaft so that the respective printing rings are resiliently held by stopper means at the predetermined starting positions with respect to the driving shaft so as to be yieldably rotated therewith. Thus, the desired one of the characters on each of the printing rings is permitted to be temporarily held at the predetermined printing position by selectively stopping the respective printing rings by the operation of arresting means during the rotation of the driving shaft against the action of the springs so as to form a line of printing of the desired characters on a paper by the cooperation of pad means adapted to be abutted against the printing rings at the printing position during each printing cycle. In order to allow the relative rotation between the respective printing ring and the driving shaft in the prior art printers, an elongated arcuate hole is provided in each of the printing rings through which a pin secured to the driving shaft is movably passed so as to connect the spring between the respective printing ring and the driving shaft for resiliently rotating the former with the latter. Alternatively, friction plate means is provided between the driving shaft and the respective printing ring so as to allow the return of the latter to the initial starting position after it has been released from the temporary arresting of the same. Such a construction is very complicated and very expensive in manufacture while the strength and the accurate operation are deteriorated.
In the prior art line printer, it is difficult to uniformly apply ink to all the characters in each printing ring by using an ink roller, because the rotational angle of the respective printing ring is different from the time at which the printing rings commence rotation for the selection of characters to the time at which the printing rings are temporarily arrested to position selected character in each printing ring at the printing position so that the rotational angle of each printing ring for returning the same to the initial starting position is different.
The present invention aims at avoiding the above described disadvantages of the prior art line printers.